1. Field of the Invention
The present invention relates to an image forming system for forming an image on a recording sheet, and a process cartridge.
2. Related Background Art
In the past, in an electrophotographic system (as an example of image forming systems), a corona charger was used as a charger means. However, the corona charger has a drawback in that not only does it require requires a high voltage but also a cleaning means for the corona wire.
To the contrary, charger means of a contact type using a conductive roller, conductive blade and the like have recently been put to practical use. It has been ascertained that such charger means provide an advantage in that they do not require either a large electric power source with a low pressure process or special cleaning means. In such charging means of contact type, the charging potential is generated on an object to be charged by contacting a conductive charger member with the object to be charged and by applying a voltage so as to cause a discharge in a gap between the charger member and the object to be charged. The object can be uniformly charged by applying to it a voltage obtained by overlapping an AC voltage to the DC voltage corresponding to the charging potential.
More specifically, as disclosed in U.S. Pat. No. 4,851,950, the object to be charged can be uniformly charged by producing an alternating electric field having a peak-to-peak voltage more than twice that of the charging start voltage upon application of the DC voltage to the object to be charged, between the charger member and the object to be charged.
Next, conventional cleaning means will be explained. In the cleaning means of the electrophotographic system, generally the non-transferred residual toner remaining on a photosensitive member (image bearing member) is scraped from the photosensitive member by rotatingly contacting a cleaning roller with the photosensitive member or by abutting a cleaning blade against the photosensitive member. Particularly, in electrophotographic systems using process cartridges, in many cases, a cleaning blade made of urethane rubber is urged against the photosensitive member in a counter direction e.g., opposite to a rotating direction of the photosensitive member, in consideration of the simple construction and inexpensiveness.
However, when a cleaning blade is used, if the friction force between the blade and the photosensitive member becomes great, the blade will often be turned over to cause a so-called "blade turn-up". When toner remains at an edge of the blade, the blade is seldom turned over because the toner acts as lubricant. However during the initial operating condition of the electrophotographic system and(or) process cartridge, since the toner does not exist at the blade edge, blade turn-up will frequency occur.
To avoid this, in the past, the friction force between the photosensitive member and the cleaning blade was reduced by applying powder to the edge of the cleaning blade at the initiation of operation of the electrophotographic system and(or) process cartridge. Such powder must have the properties that it is not harmful or poisonous, that it has a particle diameter effective to prevent blade turn-up, that it can easily be dispersed in the solvent, and that it has excellent solvent resistance. Thus, a powder comprised of PVDF (polyvinylidene fluoride) having the chemical equation of "--(CH.sub.2 CF.sub.2).sub.n --", such as, for example, "KYNAR", "FORAFLON", (both Trade Mark) was mainly used. Although the PVDF has a particle diameter of about 0.1-0.2 .mu.m, the secondary particle diameter thereof becomes 20-50 .mu.m when aggregated, and, even when ethyl alcohol is used as the solvent, it is well dispersed in the solvent and has a good painting ability and is not dissolved in the ethyl alcohol. Thus, the PVDF was widely used as the painting or coating material for the cleaning blade.
When the PVDF particles are applied to the cleaning blade abutting against the image bearing member in the charger means of contacting type, the contacting charger member must be arranged at a downstream side of the cleaning blade in a shifting direction of the photosensitive member, in consideration of the construction of the electrophotographic system. Thus, the PVDF particles passed through or excessively applied to the cleaning blade are dropped on the photosensitive member, with the result that it is feared that the dropped particles are adhered to the downstream contacting charger member. In particular, since the PVDF particles have a high aggregativity and tend to adhere to nylon resin widely used as the surface coating material for the contacting charger member, once the PVDF particles were adhered to the surface of the contacting charger member, there arose a problem in that the particles were difficult to remove even by the sliding contact between the charger member and the photosensitive member. Further, since the PVDF material is a high resistive material having an inherent volume resistance value of 10.sup.12 .OMEGA.cm or more, if such material is adhered to the surface of the contacting charger member, it is feared that the surface portions of the photosensitive member contacting with the surface of the charger member to which the material is adhered cannot be sufficiently charged, thus resulting in the poor charging.
The above drawbacks inevitably occur in systems using a contacting charger member and a cleaning blade contacting the image bearing member. Now, an additional cleaning means maybe provided for removing the PVDF particles from the contacting charger member. In this case, however, it maybe feared that the contacting charger member is damaged due to the sliding contact between it and the cleaning means and the whole system will become more expensive.